On the fourth anniversary of the invasion, Baghdad experienced at least six bomb explosions, with up to 20 dead and dozens of injuries. Much of Iraq is still in a state of violence and fear.

The war was sold to the British people on the basis that intelligence reports showed Saddam Hussein to be a serious threat.

At least three people were killed by this car bomb in Baghdad.

On 24 September 2002 Tony Blair told the House of Commons: "The intelligence picture they paint is one accumulated over the past four years.

"It is extensive, detailed and authoritative. It concludes that Iraq has chemical, biological weapons and continues to produce them."

Now, though, a very senior figure in Whitehall, heavily involved during the run-up to the war, has told me privately it was one of the great regrets of his career, with the benefit of hindsight, that he didn't challenge how the intelligence was used.

Saddam's secret

I've gathered from government sources over a period of time that British intelligence had two or three agents on the fringes of Saddam Hussein's inner circle here.

They would have sent their reports to London by radio, and must have been remarkably brave men.

But they weren't close enough to Saddam to know the best-kept secret of his rule: that at some stage in the 1990s, he got rid of most of his weapons of mass destruction.

But why should he want to keep that a secret? British officials believe it's because he was afraid his neighbour, Iran, would take advantage of his weakness, and invade.

It has emerged that MI6, the Secret Intelligence Service, was up-front about its lack of first-class intelligence about Iraq.

It told Tony Blair it hadn't known much about Iraq's work on chemical and biological weapons since 1988.

But that wasn't the impression Mr Blair gave to Parliament. As we've seen, he called the intelligence "extensive, detailed and authoritative".

'Killing questions'

In the UK House of Lords last month, Lord Butler, who headed an investigation into the intelligence which took Britain into the war, was scathing about that - though his speech went largely unreported in the media.

He accused Tony Blair of being "disingenuous" in the way he used intelligence - Whitehall-speak for "deliberately misleading".

"[Mr Blair] told Parliament... that the picture painted by our intelligence services was 'extensive, detailed and authoritative'. Those words could simply not have been justified by the material that the intelligence community provided to him."

But back in July 2004, when Lord Butler's committee issued its report, its criticisms weren't nearly as strong as that.

If they had been, that might have brought down the government - but it didn't happen.

A member of the Butler committee, the Conservative MP Sir Michael Mates, said he was surprised that no one asked what he called "the killing questions" at the press conference when the committee's report was published.

"The media didn't home in on that. Once we hadn't provided them with a scalp, they more or less lost interest."

What really saved the government was the fact that the British intelligence community had agreed to what Tony Blair told the Commons.

Senior officials may have their regrets now about the fact the intelligence was wrongly used.

But it was the discreet unquestioning support from Whitehall which meant the government was able to massage the intelligence in order to bolster its case.